Too often, unfortunately.  For example, in a 5v5 game most of the players on the losing team will realize that their team cannot win the game and then call it "gg" and quit.  But 1 or 2 people on the losing team will insist that it isn't oven (in spite of having lost 3-4 allies) and will drag things out for another half-an-hour before finally relenting.  What sucks is that at these late points in the game it's liable to be lagging, it's now boring, and some of the players might want to start a new game.

There's an alternative solution to that, just quit off the team that won.

This would also be very helpful for single-player gamers fighting against the AI, giving them a way to obtain "honourable" victory once they have achieved thorough domination.  I don't think you'd need to make an exception for dead asteroids.  They're sufficiently rare as to not be an overwhelming factor either way, and still represent a tactical asset.

This would make a great victory condition for Rebellion.  I believe I suggested something similar in my big suggestion thread a while back.